The present invention relates to a method for electronically influenced brake pressure control for a motor vehicle pressure medium-actuated power brake system, activated by a brake pedal, and, more particularly, to a brake pressure control method assigning a specific desired vehicle deceleration within a braking process to each travel of the brake pedal, and selectively setting the required brake pressure automatically by incremental adjustment of a variable proportionality factor between deceleration and brake pressure from an initial value.
A known method as described in DE 38 29 951 C2 carries out a load-dependent brake pressure control on a utility vehicle equipped with an antilock brake system. In contrast to systems with automatically load-dependent brake control in which a specific brake pedal travel is interpreted by the control as a setting of a specific brake pressure level, a specific brake pedal travel in the known method is interpreted as a setting of a desired deceleration of the vehicle. The use of an electronically controlled brake system permits the brake pressure level to be changed, even when the same brake pedal position is maintained, and, the desired deceleration to be set, specifically independently of the load state, including a possible trailer vehicle, and independently of the presence of a positive or negative gradient. This change in brake pressure with a fixed pedal position is effected by adjusting the proportionality factor between the desired deceleration and the average brake pressure. With this known measure, a satisfactory division into increments of the brake deceleration at the pedal is achieved for all load states, even taking into account the mass of a possible trailer. A uniform brake pedal sensation is achieved with a brake system in proper working order even in sections of the roadway with a positive or negative gradient. Since a genuine deceleration of the vehicle is assigned to each actuation of the pedal, the driver is also prompted, when going downhill, to use the wear-free and the fading-free permanent brake system provided for this purpose, utilizing engine brake and retarder, because only in this way is an equilibrium state with constant travel speed possible, with the possibility of intermittent braking, e.g. on a slippery or wet roadway, being retained.
Further methods in which the brake pedal travel is interpreted as a specification of a desired vehicle deceleration and the brake pressure is set correspondingly are disclosed, for example, in DE 24 30 448 A1, WO 83-03230, U.S. Pat. No. 4,712,839 and U.S. Pat. No. 4,850,650.
An object of the present invention is to provide a method with which a warning effect, in the case of an excessively low braking effect, is issued to the driver despite the automatic setting of the brake pressure.
This object has been achieved in accordance with the present invention by a method in which the adjustment of the proportionality factor is limited to a range bounded by a lower limit value and an upper limit value, and contains the initial value.
Adjustment of the proportionality factor between deceleration and brake pressure is thus limited to a predeterminable range. This leads to the proportionality factor only being adjusted at maximum up to the upper range limit value during the attempt within a braking process to achieve the desired deceleration of the vehicle, so that the brake pressure which can be achieved within this adjustment process is only increased to a specific degree above the expected value. In situations in which the brake pressure which is increased to this extent is not sufficient to achieve the desired deceleration, the brake pressure consequently remains at the value associated with the upper limit value of the proportionality factor and the actual deceleration achieved is consequently below the desired deceleration. This insufficient deceleration can be detected by the driver to provide a warning of excessively low braking effect.
A further increase in brake pressure with the effect of achieving the deceleration of the vehicle desired by the driver requires in any event an additional brake pedal travel which makes the driver aware of the excessively weak braking effect. This offers the possibility of a drivel intervention in the braking process by a further actuation of the brake pedal in order to achieve the desired deceleration without the brake pressure having to be automatically adjusted over an excessively large range.
The present invention results in a brake pressure adjustment which is symmetrical on both sides. The control algorithm is additionally simplified since it is only necessary to specify one limit value interval from the initial value for both deviation directions.
According to another aspect of the present invention, the detection of the exceptional case of emergency braking is permitted in which the limiting of the adjustment of the proportionality factor is eliminated so that the procedure of limited adjustment of the proportionality factor with the warning effect because of fading as a result of a negative gradient or a braking effect which decreases for another reason remains limited to the range of relatively small average brake pressures.
An embodiment of the present invention provides a particularly clear warning effect since, as a result of the returning of the proportionality factor from the limit value to the initial value at the start of a further actuation of the brake pedal, the braking effect does not initially change in this initial stage of the further actuation of the brake, or at any rate does not change to a degree proportional to the travel of the pedal, so that a genuine warning pedal travel is produced for the driver without increasing the deceleration of the vehicle.
As soon as the proportionality factor has reached the initial value, the further actuation of the brake pedal then leads again to a corresponding increase in the brake pressure until the desired deceleration of the vehicle is achieved.
In two embodiments of the present invention, the limit value interval of the proportionality factor of the initial value is prescribed as a function of the initial value itself or of the desired deceleration of the vehicle. This feature particularly takes into account fluctuations in the initial value which are to be expected depending on the driving speed, temperature and negative gradient and permits the realization of a clear warning travel even at the start of a journey and above all more reliably in the case of small desired decelerations than in the case of the selection of a constant limit value interval which is independent of these parameters.
In a configuration of the invention, a new evaluation, that is an adjustment, of the initial value for the proportionality factor for subsequent braking processes can be carried out from the value of the proportionality factor determined for the instantaneous braking, specifically dynamically, i.e. to a comparatively high degree and quickly, given an initially very high deviation of the actual deceleration from the desired deceleration of not less than +/xe2x88x9230%, and semi-dynamically, i.e. to a comparatively low degree and in small increments if a residual deviation of the actual deceleration from the desired deceleration by more than 3% above or below is detected. The subsequent braking process thus starts already with an improved initial brake pressure value, which generally reduces the duration of the necessary adjustment.